1. Field of the Invention
This invention relates to an internally cooled rotor winding for a dynamoelectric machine, and more particularly to a salient pole rotor winding for a hydrogenerator.
2. Description of the Prior Art
The economics of power generation in recent years has called for a reconsideration of the design of generators in order to meet higher operating parameters and efficiencies.
Large hydrogenerators are typically of a salient pole rotor construction. The pole winding comprises helically wound conductors. During operation of the machine the conductors must be cooled. Two types of cooling systems in wide use are forced air ventilation and liquid cooling. In liquid cooling systems, the conductors are cooled by circulating a coolant liquid through either a duct within the main ground insulation or a bore within the conductor itself. This method of cooling improves the heat transfer efficiencies of the machine, and is therefore preferred in large hydrogenerators over other types of cooling such as forced air cooling.
Forced air ventilation systems had at least two drawbacks. First, the air was circulated between the salient poles, in what is called the interpolar space; so the heat transfer requirements, as well as the electromagnetic properties, of the machine dictated the minimal dimensions of this space. Second, electrical insulation around the conductor acted as a thermal barrier and decreased heat removal capability.
The utilization of internally cooled conductors obtains the following benefits: the interpolar space can be reduced in size, with the resultant increased space for conductor packing; the enlarged copper cross-section reduces excitation losses by lowering current density; and the circulated water requires less pumping power than does air, and yet is a more efficient coolant.
A problem with internally cooled salient pole rotors is access to the lead-outs of the conductors. Frequently, hydrogenerators are installed in a vertical manner, that is, with the generator above a turbine. If the ends of the conductors are brought out on opposite sides of the rotor, hydraulic interconnection would have to be made, for example, by running piping through the rotor shaft to reach the leadouts on the side of the rotor facing the turbine. This may prove difficult due to the proximity of other machine structures. Also, this may present repair and maintenance difficulties. Desirable, therefore, is the design of a winding in which all leadouts are on the same side of the rotor; the opposite side from the one facing the turbine. Also desirable would be a design that permits all lead-outs to be clamped together at the interconnection, thereby increasing their mechanical strength.
In the design of a pole winding having the lead-outs on the same side of the rotor, three design trade-offs must be kept in mind. These considerations are: (1) the arrangement of the winding to achieve access to all the conductors of a multi-layered winding; (2) the manufacturing difficulties in fabricating and bending conductors into a helix without distorting the geometry of the conductor, or the bore in the bend region; and (3) the optimum utilization of the pole cross-sectional area for conductor packing.
Conductor packing is important because the more efficiently the available winding area is utilized, the more conductor can be wound around the shank of a pole of given dimensions. A rotor's ratings can be increased by increasing the efficiency of the conductor packing around a pole.
The conventional conductor geometry is generally rectangular in cross-section of large aspect ratio, for example having dimensions of approximately one-quarter inch by four inches in width. Thus the conductor is "strap" or "sheet" shaped. In machines having direct coolant systems the conductors may have a bore centrally located. The conductors may be formed into a helix by bending the hollow conductor edgewise along their wider dimension. This bending may distort the cross-sectional area and the bore's shape. In lieu of bending and to preserve the integrity of the geometry, the dynamoelectric machine's helixes of conductor have previously been formed by brazing a multiplicity of short lengths of conductor together. The brazed joints on occasion may lead to leakage problems when this method is used for internally cooled machines.
In a conventional salient pole, the wrapping of a helical winding around the shank of the pole necessitates the use of conductors having large aspect ratios in order to minimize vacant, unused space which is available for winding. As the thickness of the conductor's narrower dimension increases the amount of unused available winding area also increases. One reason this occurs is that in a known winding arrangement the ends of the helical winding proceed only part way around the pole shank so that they may be interconnected at a predetermined location about the pole. The thicker the conductor's narrower dimension, in such a winding arrangement, the more available space is left unused, and the lower the conductor packing efficiency.